Collection types in databases refer to data types that can store multiple values or elements in a single column. They're often used when an attribute is multivalued, i.e., can take on more than one value at a time. It's worth noting that not all databases support collection types, and how they are implemented can vary widely.
Three common types of collection types are:
Arrays: An array is an ordered list of elements, all of the same type. It is indexed, meaning you can access individual elements using their positions in the array. PostgreSQL, for instance, supports arrays as a data type.
Sets: A set is a collection of unique elements. Unlike arrays, sets are not ordered, so they do not have indexes. In terms of databases, some NoSQL databases like MongoDB and Cassandra use sets in their data models.
Multisets or Bags: A multiset (or bag) is similar to a set but allows duplicate elements. Like a set, a multiset is not ordered.
In addition to these, some databases support more complex collection types, such as nested tables (tables within tables) and varying arrays (arrays that can change in size).